Process of hot dip tinning



Dec. 5, 1950 H. c. RODGERS PROCESS OF HOT DIP TINNING Filed July 30,1947 air/m 6 m A K fl w Patented Dec. 5, 1950 PROCESS OF HOT DIP TINNINGHoward C. Rodgers, Birmingham, Ala., assignor to Tennessee Coal, Ironand Railroad Company, a corporation of Alabama Application July 30,1947, Serial No. 764,720

Claims.

This invention relates to a process of hot dip tinning and moreparticularly to such a process for tinning steel sheets. In the ordinarymethod of hot dip tinning of steel sheets the sheets are loaded in batchpickler crates and sloshed up and down in sulphuric acid baths. Toprevent sheets from being over-pickled, inhibitors such as flour, salt,wheat middlings and organic inhibitors are added to the bath. Thepickled sheets are then transferred to a rinse tank in which circulatingcold water washes off the sulphuric acid and the inhibitor. When thesheets are unloaded from the crates they are stacked on edge in a tankand covered with water having a small amount of hydrochloric acidtherein. The tank is then transferred to the tin pot by a tractor andthe sheets are manually transferred by hand to another water tank. Thesheets are fed by a magnet out of this last tank through several sets ofrolls and are guided downwardly through the zinc ammonium chloride fluxwhich is floating on the bath of molten tin. The temperature of the fluxis approximately 350 F. and that of the tin is between 600 and 650 F.The flux adheres to the sheets and is carried downwardly into the moltentin which raises its temperature. As the flux on the sheets is raised intemperature, it attacks the steel sheets which accelerates the formationof an iron tin alloy layer. The sheet then passes through various setsof rolls and guides in a tinning machine and through a bath of oilhaving a temperature of 465 F. The coating is solidified by low pressurecompressed air and the sheet is then cleaned and polished. This processresults in dull gray areas on the sheets which are known in the trade asleopard spots. These areas are usually circular in appearance and have adiameter of between /4. and 1". In extreme cases these spots cover largeareas on the sheets and usually occur in areas where the coating 'isthin. The exact cause of the dull gray areas is not definitely knownthough countless technical articles have been written on the subject.Prior to my invention no one had discovered a way to prevent theformation of leopard spots without interfering with production. Thesespots are detrimental to the appearance of the sheets and the quality ofthe product. It is my belief that these spots are caused by the fluxadhering to the sheets being raised to excessive temperatures in the tinpot.

It is an object of my invention to provide a process of hot dip tinningwhich prevents these leopard spots from being formed.

This and other objects will be more apparent after referring to thefollowing specification and attached drawings, in which:

Figure 1 is a diagrammatic view of the tin line used in carrying out minvention; and

Figure 2 is an enlarged sectional elevation ofthe pickling tank.

Referring more particularly to the drawings, the reference numeral 2indicates a pair of feed rolls, the top roll being made of rubber whilethe bottom roll is made of steel. These rolls are located in a rubberlined pickling tank t having a pickling solution 6 therein. Thissolution may be hydrochloric acid, sulphuric acid, ammonium chloride, orany other pickling solution, all. of which are well known in the trade.However, it is preferred to use hydrochloric acid since it is compatiblewith the flux which is made from hydrochloric acid, salammoniac andzinc. The preferred solution has an acid concentration of betweenapproximately .25 to 5.0% with excellent results being obtained with aconcentration of approximately 1%. The solution also contains from .01%to .06% of an inhibitor. Any of the standard inhibitors may be used, butI prefer to use those disclosed in the patents to Cunningham Nos.1,961,096 and 1,961,097. The pickling solution is used for approximatelya week after which the tank is drained, rinsed and refilled with a newpickling solution. The pickling may be accomplished with or without theintroduction of electric current, but electrolytic pickling will bedescribed. Adjacent the inlet side of the tank 4 is a stainless steelconductor roll 8 having a rubber roll it bearing thereagainst. Spacedfrom the roll 8 is a second stainless steel conductor roll l2 alsohaving a rubber roll I4 bearing thereagainst. Between the rolls 8 and I9are top and bottom sheet guides l6 and I8, respectively. Mounted on thetop guide I6 is a steel anode, 29 and mounted on the bottom guide is acarbon anode 22. A guide 24 extends upwardly from the roll l2 toward apair of rubber rolls 26 which feed the sheets from the tank 4. Anoverflow pipe 28 controls the level of the solution in the tank 4 and apetcock 30 is provided to keep the solution up to the top of theoverflow pipe. Adjacent the rolls 26 is a pair of rubber rolls 32mounted in a water tank 34, the top roll of which is preferably grooved.Between the rolls 26 and 32 is a water spray 39. From the rolls 32 thesheets S pass through a feeder 38 to the tinning machine 40 whichcontains a flux 42 floating on the molten tin 44. The sheets S asuitable feeder such as the Dexter metal sheet j-.

feeder and through the rolls 2 into the bath 6. The sheets then passthrough rolls '8 and HP, guides 19 and it, rolls i2 and [4, guide 24 androlls 26. As the sheets pass through the tank they receive the positivecurrent which flows from the anodes through the pickling bath to thenegative conductor rolls so that the sheets become cathodic. Picklingcurrent having a range of from 100 to 500 amperes is supplied by a D. C.motor generator set with the current being maintained between 4 to 12volts. The rubber rolls 26 squeeze the excess pickling solution from thesheets. Since the sheets are dry when they are fed into the picklingsolution, the surface of the sheets readily absorbs the inhibitor whichis carried in the minute pores in the. steel through the water spray 36directly into the flux floating in the molten tin bathf The water spray36 merely serves to keep the surface of the sheet wet and while it mayWash oil of a small part of I the inhibitor, sufficient inhibitorremains on the sheet to protect it. A sheet 28 /2 inches long will passentirely under the water spray and through the rubber rolls in 4seconds, thus limiting the rinsing action and allowing the major part ofl the inhibitor to be carried over into the tin pot. The same size sheetwill be in the pickling solution approximately 9 seconds. The sheetpasses from the tin pot 40 in the usual manner and is washed and passedthrough the branner. In the process it is very important that thepickled sheets are wet going into the flux and that the sheets leavingthe pickling bath are not washed so that the inhibitor remains on thesteel as it goes into the flux and protects the steel from attackthereby.

It will be apparent that other modifications and adaptations may be madeWithout departing wetted sheets with inhibitor thereon through tinningflux floating on the tin directly into the tin, and applying tin to thesheets.

2. The process of hot dip tinning of ferrous metal sheets whichcomprises passing the sheets through a pickling solution having apickling inhibitor therein, removing the sheets in a wetted conditionfrom the solution with pickling solution and inhibitor thereon, passingthe wetted sheets under a water spray, passing the wetted sheets withinhibitor thereon through tinning flux floating on the tin directly intothe tin, and applying tin to the sheets.

3. The process of hot dip tinning of ferrous 4' metal sheets whichcomprises passing the sheets through a pickling bath of hydrochloricacid having a pickling inhibitor therein, removing the sheets in awetted condition from the bath with hydrochloric acid and inhibitorthereon, passing the wetted sheets with inhibitor thereon throughtinning flux floating on the tin directly into'the tin, and applying tinto the sheets.

4. The process of hot dip tinning of ferrous metal sheets whichcomprises passing the sheets through a pickling bath of hydrochloricacid having a pickling inhibitor therein, removing the sheets in awetted condition from the bath with hydrochloric acid and inhibitorthereon, passing the wetted sheets under a water spray, passing thewetted sheets with inhibitor thereon through tinning flux floating onthe tin directly into the tin, and applying tin to the sheets.

5. The process of hot dip tinning of ferous metal sheets which comprisespassing the sheets through a pickling bath having hydrochloric acid ofapproximately 1.0% concentration and from approximately .01% to .06%pickling inhibitor, removing the sheets in a wetted condition from thebath with hydrochloric acid and inhibitor thereon, passing the wettedsheets under a water spray, passing the wetted sheets with inhibitorthereon through tinning flux floating on the tin directly into the tin,and applying tin to the sheets.

HOWARD "C. RODGERS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS lhnnher Name Date 1,961,096 Cunningham May 29,1934 1,961,097 Cunningham May 29, 1934 2,223,355 Gonser Dec. 3, 1940

1. THE PROCESS OF HOT DIP TINNING OF FERROUS METAL SHEETS WHICHCOMPRISES PASSING THE SHEETS THROUGH A PICKLING SOLUTION HAVING APICKLING INHIBITOR THEREIN, REMOVING THE SHEETS IN A WETTED CONDITIONFROM THE SOLUTION WITH PICKLING SOLUTION AND INHIBITOR THEREON, PASSINGTHE WETTED SHEETS WITH INHIBITOR THEREON THROUGH TINNING FLUX FLOATINGON THE TIN DIRECTLY INTO THE TIN, AND APPLYING TIN TO THE SHEETS.